In the reproduction of an original image using image data available in the form of a video signal or a series of pixels obtained from line by line scanning of an original such as by means of a video camera, an array of charge-coupled elements, and the like, the type of original image must be taken into account because if the original image consists wholly or partly of a half-tone image, disturbing patterns may occur in the reproduction depending upon the method of reproduction used. See European Patent Application No. 89,931 for one type of device for scanning and digitally processing half-tone images.
With regard to image information, a distinction may be made between the following: (a) text, which has a substantially regular distribution of grey level flanks, the distance between which differs according to the text in question, but is generally on the order of 3 mm; (b) line drawings, which have steep grey level flanks with an irregular rather than regular distribution, the distance between the flanks may be less than in the case of text, on the order of 0.5 mm; (c) continuous tone images (i.e. photographs) where the grey levels usually vary slowly and gradually, so that the flanks are less steep with no regular distribution; and (d) half-tone images or dither images, which frequently have a regular pattern of grey level peaks with slowly varying peak height, however, the local frequency at which these peaks occur is high and therefore the distance between the peaks is relatively low, on the order of a few tenths of a millimeter.
When an image is reproduced by means of a dither process, moire patterns may occur at the places where half-tone image information is present in the image and this is very undesirable. On the other hand, by means of special reproduction techniques known per se and intended more particularly for the processing of half-tone image information, it is possible to avoid the occurrence of such disturbing patterns. For image information to be processed during the reproduction process in the correct way depending upon the type of information, it is necessary to know what part of the complete image information area contains half-tone image information. There is therefore a need for a method by means of which a distinction can be made between half-tone image information and other types of image information.
A method and a device for distinguishing between half-tone image information and other types of image information is described in U.S. Pat. No. 4,194,221. There, a video signal originates from an image scanner and available in the form of a series of pixels at the output of the CCD (charge coupled device) scanner. This signal is temporarily stored in a shift register and is subjected to an autocorrelation process in which an autocorrelation function is calculated in each case for a predetermined number of successive pixels. In this function the peak values are detected and finally the distances between these peak values are determined. If these distances are less than a predetermined threshold value or, in other words, if the frequency of the autocorrelation function is higher than a specific threshold value, the decision is taken that the relevant number of pixels represents high-frequency half-tone image data.
This known device has a number of disadvantages. Calculating the autocorrelation function for each of a series of pixels, detection of the peak values therein, and determination of the distances between these peak values is relatively time consuming and requires a considerable memory capacity in the device with which the method is performed. Thus a considerable amount of time is required to process all the information of the original image and this means that scanning of the original must take place relatively slowly or, if the original is scanned quickly, a relatively large memory is required to be able to store, at least temporarily, the video signal or the generated series of pixels.
It would be desirable to provide a method which is simpler than this known method, requires less complex calculations and can be at least approximately performed in real time even where fast scanning of the original occurs. It also would be desirable to provide a device for performing a method of this kind without the need for large capacity memories and which can be implemented with relatively simple means both in the form of a hardware circuit and in the form of an appropriately programmed microprocessor with peripheral circuits.